This invention relates to a signal processing system for generating a clock signal synchronized to a video information signal, and, in particular, to input clock generators for time base correctors and synchronizers utilized in processing video information signals.
In the reproduction of television video signals from information recorded, for example, on magnetic tape or magnetic discs, as well as from other local or network sources, it is often found that the video signals require time base correction.
In a time base corrector, as shown in U.S. Pat. No. 3,860,952, the incoming video signal is converted into digital form by means of an analog to digital converter and stored in a memory storage means. Time base errors are removed by reading the digitized video signal into the memory storage means at a clocking rate which is made dependent on the time base errors occurring in the incoming video signal. The video signal read into storage at the variable rate is read out of the memory storage means and converted into analog form at a constant clocking rate determined by a local external reference. The variable clocking rate for time base correctors of the type described is generally derived from a voltage controlled oscillator (VCO), the output frequency of which is made dependent on the instantaneous time variations of the incoming video signal. However, it is difficult to generate a clock frequency using a VCO that will be both fast acting and still remain stable for the required time period between television lines. This is particularly true in processing color television signals where the signal perturbations of the color subcarrier burst signal are utilized to shift the output frequency of the VCO.
The normal burst signal of approximately 2.mu.sec (8 to 9 cycles of 3.58 MHz in the NTSC television system) is too short a time for a consistently reliable measurement of the burst phasing and to enable the subsequent VCO correction to match the oscillator frequency phase with the incoming burst signal phasing. Attempts at increasing the speed of the oscillator phase capture time generally tends to decrease the stability of the oscillator output. If the oscillator stability is increased by increasing the oscillator time constants, then several burst signal intervals, i.e., more than one television line period, may be needed to bring the oscillator phasing into the correct phase relationship for properly storing the color video signal. Compromising or balancing oscillator capture speed versus oscillator stability, although helpful, does not completely solve the problem where, for example, the next burst signal may not be available due to signal degradation or where the burst signal is not available as during the vertical blanking interval wherein the burst signal is not produced for nine consecutive television lines. When the burst signal does reappear, it may take the oscillator several television lines to become properly phased with the incoming video signal.
As previously described in connection with the time base corrector illustrated by U.S. Pat. No. 3,860,952, video signals stored in the memory storage means are read out of storage at a constant clocking rate by a stable local reference frequency. This local reference is continuous, of constant subcarrier frequency, and has the desired stability. However, it is not usable directly for the input clock generator frequency since reading into memory must be accomplished at a rate which varies with the time base error of the incoming video signal to achieve time base correction. In accordance with the present invention, the external subcarrier reference signal may be utilized for deriving a variable input clock frequency if the external subcarrier is rephased in accordance with the time perturbations of the incoming video signal. One means of rephasing a subcarrier reference frequency is shown in U.S. Pat. No. 4,001,876 in which a chroma signal is demodulated by a reference signal having the same time base errors as the chroma signal. The demodulated chroma signal is then remodulated onto a stable subcarrier for recombination with the luminance signal to provide a color corrected video signal.
In the present invention, the subcarrier burst signal of an incoming video signal having time base errors is demodulated by a constant frequency reference and the detected error utilized to re-modulate the identical reference to obtain an instantaneously rephased subcarrier which will provide a variable input clock frequency with all the stability of the original reference signal.